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Mössbauer, optical absorption and EPR spectra of Fe3+ in plumbo-jarosite minerals
Physics Letters A 16 I North-Holland
( I99 1) 74-76
PHYSICS
Mijssbauer, optical absorption in plumbo-jarosite minerals S. Lakshmi
LETTERS
A
and EPR spectra of Fe3+
Reddy ‘, P.S. Rao b and B.J. Reddy a
Received I I July 199 I; revised manuscript Communicated by J.I. Budnick
received
I October 199 I
MGssbauer spectrum of plumbo-jarosite mmeral from Utah showed the presence of Fe(II1) in a distorted octahedron. EPR investigations of the sample further confirmed that ferric Iron is in a distorted octahedron. The optical absorption spectrum consists of a series of bands that can be assigned to spin-forbidden crystal field transitions of Fe( 111) in octahedral coordination.
1. Introduction Alunite-jarosite group minerals have the general formula MA,(S04)2(0H),. where M is Na, K, Pb, Ag. etc., and A is Al for the alunite group and Fe for the jarosite group [ I 1. The crystal structure of plumbo-jarosite, Pb[Fel(S04)1(0H),]Z, obtained from Utah has been reported [2] and reveals that the Fe atom is in a distorted octahedron surrounded by four hydroxyl groups in the equatorial plane and two oxygen ligands in the axial positions. In order to elucidate the site distribution of ferric ions, we report the results of MGssbauer, EPR and optical absorption spectroscopic features on plumbo-jarosite mineral originated from Tintic-Standard Mine, Utah. USA (gifted by Dunn, Smithsonian Institution. Washington DC with catalogue number 104271).
2. Experimental methods Mijssbauer spectra were obtained on an ECIL Miissbauer spectrometer with a multichannel analyser in the time mode and krypton-filled proportional detector using a “Cu, 57Co source in palladium matrix. Electron paramagnetic resonance (EPR) spectra of powdered plumbo-jarosite mineral were taken at room temperature with a Varian E- 112 spectrometer operating at about 9.5 GHz, having 100 74
0375-9601/91/$
kHz field modulation and phase sensitive detection to obtain a first-derivative EPR signal. DPPH was used as an internal field marker. The optical absorption spectra were recorded at room temperature on a Varian Cary 2390 spectrophotometer using the nujol mull in the region 200 to 1200 nm.
3. Results and analysis 3. I. ,~4iisshaue~ studies The MBssbauer spectrum of plumbo-jarosite mineral recorded at room temperature shown in fig. 1. exhibits a doublet with isomer shift (IS) 6=0.4763 mm/s and quadrupole splitting QS= 1.19 mm/s. This data, similar to that of jarosite minerals of different origins reported earlier [ 3 1, confirms that the iron is in the Fe( III) state. In pyroxenes, micas. garnets and amphiboles the IS of ferric iron in octahedral coordination is reported to be 0.20 mm/s and the value of QS lies in the range of 0.40 to 0.60 mm/ s [ 41. In the present sample the higher value of QX of ferric doublet confirms the Fe (III ) ion to be in a distorted octahedron. 3.2. EPR tneusuwt~wnts The powdered EPR spectrum of plumbo-jarosite mineral is shown in fig. 2. The lines marked with an 03.50 0 I99
I Elsevier Science Publlshcrs B.V. All rights reserved.
Volume 16 I. number
I
PHYSICS
LETTERS
I6 December
A
I99 1
broad line beneath the Mn(I1) sextet is also due to Fe( III) impurity, appearing due to the population of one of the excited Kramers doublets [ 5 1.
I---
tL
Fig. I. MGssbauer rosite mineral.
hypertine
Fig. 2. Room temperature rosite (v=9.368 GHz).
absorption
powder
spectrum
of plumbo-ja-
EPR spectrum
of plumbo-ja-
asterisk have g values 4.346, 3.286, 2.929 and 2.406. These can be attributed to Fe( III) in the sample. As can been discussed in a previous communication [ 51, if the Fe(III), d5 system is under the influence of a strong tetragonal distortion, the three Kramers doublets will have g values ranging from 4.30 to 0.80. In addition to Fe( III), the sample contains a very low concentration of Mn (II) (the six weak lines centred around DPPH, marked with a double asterisk). The
3.3. Electronic processes The electronic configuration for iron is A(3d)‘, where A stands for zero closed argon shell. It gives rise to free ion terms ‘S, 4G and several quartet and doublet states in octahedral symmetry. In the octahedral crystal field ?S transforms as 6A,,(S) whereas 4G splits into 4A,,(G), 4E,(G), 4Tz,(G) and 4T,,(G), among which 6A,,(S) is the lowest, being the ground state. Fig. 3 shows the optical absorption spectrum of plumbo-jarosite mineral in the range 200- 1200 nm. The spectrum discussed here has all the features that appear due to various electronic transitions involving iron. The optical spectra of the components which are known to contain only linear chains of hydroxobridged Fe(III) display similar spectra. For Fe(III), there are three. features with transitions 6A,,(S) + 4T,,(G) (v,) which occurs between 10525 and 13300 cm-‘, the ‘AIg(S)d4TZg (v,), which appears from 15380 to 18 180 cm- ’ usually as a shoulder and 4E(G) ( u3) which appear the 6A,,(S)+4A,,(G).
xl
Fig. 3. Optical absorption
spectrum
of Fe( III) in plumbo-jarosite.
75
Volume
I6
I, number I
PHYSICS
around 22000 cm-‘. The last transition is field independent [ The ligand field spectrum of ferric iron appears as if only the first (v,) and the third ( v3) bands of octahedral symmetry are present. The analysis of the general features of the spectrum of hydroxobridged chains of Fe( III) are reported on sulphate bearing minerals [ 7,8 1. These data have been used to identify the bands due to Fe( III) in plumbojarosite. Accordingly, the first feature observed in the range 12000-I 5500 cm-’ is attributed to ‘A,,(s)-4T,,(G), the third band at 22730 cm-’ is sharp and assigned to hA,,(S)+4A,,(G), 4E(G) transitions respectively. A broad and diffused band at 19045 cm- ’ is identified as the 4Tzg( G) band. The other energy bands could be assigned to the transitions with the help of the Tanebo-Sugano diagram. The assignments of the bands are given in table 1. The crystal field parameters B, C and Dq were evaluated by solving energy matrices of d5 configuration applying Tree’s correction (90 cm- ’ ) [ 91. The parameters which give a good fit to the experimental data are B=700 cm-‘, C=2800 cm-’ and Dq=900 cm - ‘. The calculated values are listed in table 1.
Table I The observed and calculated wavenumbers the bands of Fe(II1) in plumbo-jarosite. Transition from 6A,,(S)
and assignments
“T>,(G) ‘Alp(G). “T?,(D) ‘E,(D) 4T,,(P) 4Trg(F) “T?&(F)
76
4E(G)
16 December
I99
Sulphate bearing minerals exhibit two prominent bands around 15000 cm-’ and 22000 cm-’ which are characteristic of the Fe( III) ion. Similar features revealed in plumbo-jarosite confirm ferric iron in octahedral symmetry. The analysis of Mijssbauer data indicates that ferric iron is in a distorted octahedron. The mineral plumbo-jarosite shows Fe(II1) EPR resonances in which iron is under the influence of a strong tetragonal distortion. In addition, the mineral contains a low concentration of Mn( II), as is evident from the EPR spectrum.
Acknowledgement One of the authors (S.L. Reddy, S.V. Degree College, Cuddapah 5 16 003, India) is grateful to UGC, New Delhi for financial assistance. S.L. Reddy greatly appreciates the continued moral support and help of this work by A.G. Reddy, Principal, S.V. Degree College, Cuddapah, India. PSR thanks CSIR, India for a fellowship.
References calculated (cm-‘)
A (nm)
V
800 650 525 440 410 385 330 265 240
12500 15385 19045 22730 24390 25975 30300 37735 41665
v (cm-‘)
_ 15194 19379 22166 24815 26474 30656 37710 41125
1
4. Conclusions
for
Band position observed
4T,,(G)
LETTERS A
[I
] A.J. Ripmeester,
I.C. Ratcliffe. E.J. Dutrtzac and J.L. Jambor. Can. Min. 24 (1986) 435. [2] J.T. Szymanski, Can. Min. 23 (1985) 659. [3] A.Z. Hrynkiewicr. J. Kubisz and D.S. Kulgawczuk. Inorg. Nucl. Chem. 27 ( 1965) 25 13. [4] R.G. Rossman, SE. Grew and W.A. Dollase, Am. Min. 67 (1982) 749. [5 ] Subramanian, Mol. Phys. 54 ( 1985) 415. [ 61 T.R. Hunt and P.R. Ashley, Economic Geology 74 ( 1979) 1613. [ 71 R.G. Rossman, Am. Min. 60 ( 1975) 698. [ R.G. Rossman, Am. Min. 6 I ( 1976) 398. Rosengarten, Mol. Spectrosc. 12( 1964) 3 19.
( I99 1) 74-76
PHYSICS
Mijssbauer, optical absorption in plumbo-jarosite minerals S. Lakshmi
LETTERS
A
and EPR spectra of Fe3+
Reddy ‘, P.S. Rao b and B.J. Reddy a
Received I I July 199 I; revised manuscript Communicated by J.I. Budnick
received
I October 199 I
MGssbauer spectrum of plumbo-jarosite mmeral from Utah showed the presence of Fe(II1) in a distorted octahedron. EPR investigations of the sample further confirmed that ferric Iron is in a distorted octahedron. The optical absorption spectrum consists of a series of bands that can be assigned to spin-forbidden crystal field transitions of Fe( 111) in octahedral coordination.
1. Introduction Alunite-jarosite group minerals have the general formula MA,(S04)2(0H),. where M is Na, K, Pb, Ag. etc., and A is Al for the alunite group and Fe for the jarosite group [ I 1. The crystal structure of plumbo-jarosite, Pb[Fel(S04)1(0H),]Z, obtained from Utah has been reported [2] and reveals that the Fe atom is in a distorted octahedron surrounded by four hydroxyl groups in the equatorial plane and two oxygen ligands in the axial positions. In order to elucidate the site distribution of ferric ions, we report the results of MGssbauer, EPR and optical absorption spectroscopic features on plumbo-jarosite mineral originated from Tintic-Standard Mine, Utah. USA (gifted by Dunn, Smithsonian Institution. Washington DC with catalogue number 104271).
2. Experimental methods Mijssbauer spectra were obtained on an ECIL Miissbauer spectrometer with a multichannel analyser in the time mode and krypton-filled proportional detector using a “Cu, 57Co source in palladium matrix. Electron paramagnetic resonance (EPR) spectra of powdered plumbo-jarosite mineral were taken at room temperature with a Varian E- 112 spectrometer operating at about 9.5 GHz, having 100 74
0375-9601/91/$
kHz field modulation and phase sensitive detection to obtain a first-derivative EPR signal. DPPH was used as an internal field marker. The optical absorption spectra were recorded at room temperature on a Varian Cary 2390 spectrophotometer using the nujol mull in the region 200 to 1200 nm.
3. Results and analysis 3. I. ,~4iisshaue~ studies The MBssbauer spectrum of plumbo-jarosite mineral recorded at room temperature shown in fig. 1. exhibits a doublet with isomer shift (IS) 6=0.4763 mm/s and quadrupole splitting QS= 1.19 mm/s. This data, similar to that of jarosite minerals of different origins reported earlier [ 3 1, confirms that the iron is in the Fe( III) state. In pyroxenes, micas. garnets and amphiboles the IS of ferric iron in octahedral coordination is reported to be 0.20 mm/s and the value of QS lies in the range of 0.40 to 0.60 mm/ s [ 41. In the present sample the higher value of QX of ferric doublet confirms the Fe (III ) ion to be in a distorted octahedron. 3.2. EPR tneusuwt~wnts The powdered EPR spectrum of plumbo-jarosite mineral is shown in fig. 2. The lines marked with an 03.50 0 I99
I Elsevier Science Publlshcrs B.V. All rights reserved.
Volume 16 I. number
I
PHYSICS
LETTERS
I6 December
A
I99 1
broad line beneath the Mn(I1) sextet is also due to Fe( III) impurity, appearing due to the population of one of the excited Kramers doublets [ 5 1.
I---
tL
Fig. I. MGssbauer rosite mineral.
hypertine
Fig. 2. Room temperature rosite (v=9.368 GHz).
absorption
powder
spectrum
of plumbo-ja-
EPR spectrum
of plumbo-ja-
asterisk have g values 4.346, 3.286, 2.929 and 2.406. These can be attributed to Fe( III) in the sample. As can been discussed in a previous communication [ 51, if the Fe(III), d5 system is under the influence of a strong tetragonal distortion, the three Kramers doublets will have g values ranging from 4.30 to 0.80. In addition to Fe( III), the sample contains a very low concentration of Mn (II) (the six weak lines centred around DPPH, marked with a double asterisk). The
3.3. Electronic processes The electronic configuration for iron is A(3d)‘, where A stands for zero closed argon shell. It gives rise to free ion terms ‘S, 4G and several quartet and doublet states in octahedral symmetry. In the octahedral crystal field ?S transforms as 6A,,(S) whereas 4G splits into 4A,,(G), 4E,(G), 4Tz,(G) and 4T,,(G), among which 6A,,(S) is the lowest, being the ground state. Fig. 3 shows the optical absorption spectrum of plumbo-jarosite mineral in the range 200- 1200 nm. The spectrum discussed here has all the features that appear due to various electronic transitions involving iron. The optical spectra of the components which are known to contain only linear chains of hydroxobridged Fe(III) display similar spectra. For Fe(III), there are three. features with transitions 6A,,(S) + 4T,,(G) (v,) which occurs between 10525 and 13300 cm-‘, the ‘AIg(S)d4TZg (v,), which appears from 15380 to 18 180 cm- ’ usually as a shoulder and 4E(G) ( u3) which appear the 6A,,(S)+4A,,(G).
xl
Fig. 3. Optical absorption
spectrum
of Fe( III) in plumbo-jarosite.
75
Volume
I6
I, number I
PHYSICS
around 22000 cm-‘. The last transition is field independent [ The ligand field spectrum of ferric iron appears as if only the first (v,) and the third ( v3) bands of octahedral symmetry are present. The analysis of the general features of the spectrum of hydroxobridged chains of Fe( III) are reported on sulphate bearing minerals [ 7,8 1. These data have been used to identify the bands due to Fe( III) in plumbojarosite. Accordingly, the first feature observed in the range 12000-I 5500 cm-’ is attributed to ‘A,,(s)-4T,,(G), the third band at 22730 cm-’ is sharp and assigned to hA,,(S)+4A,,(G), 4E(G) transitions respectively. A broad and diffused band at 19045 cm- ’ is identified as the 4Tzg( G) band. The other energy bands could be assigned to the transitions with the help of the Tanebo-Sugano diagram. The assignments of the bands are given in table 1. The crystal field parameters B, C and Dq were evaluated by solving energy matrices of d5 configuration applying Tree’s correction (90 cm- ’ ) [ 91. The parameters which give a good fit to the experimental data are B=700 cm-‘, C=2800 cm-’ and Dq=900 cm - ‘. The calculated values are listed in table 1.
Table I The observed and calculated wavenumbers the bands of Fe(II1) in plumbo-jarosite. Transition from 6A,,(S)
and assignments
“T>,(G) ‘Alp(G). “T?,(D) ‘E,(D) 4T,,(P) 4Trg(F) “T?&(F)
76
4E(G)
16 December
I99
Sulphate bearing minerals exhibit two prominent bands around 15000 cm-’ and 22000 cm-’ which are characteristic of the Fe( III) ion. Similar features revealed in plumbo-jarosite confirm ferric iron in octahedral symmetry. The analysis of Mijssbauer data indicates that ferric iron is in a distorted octahedron. The mineral plumbo-jarosite shows Fe(II1) EPR resonances in which iron is under the influence of a strong tetragonal distortion. In addition, the mineral contains a low concentration of Mn( II), as is evident from the EPR spectrum.
Acknowledgement One of the authors (S.L. Reddy, S.V. Degree College, Cuddapah 5 16 003, India) is grateful to UGC, New Delhi for financial assistance. S.L. Reddy greatly appreciates the continued moral support and help of this work by A.G. Reddy, Principal, S.V. Degree College, Cuddapah, India. PSR thanks CSIR, India for a fellowship.
References calculated (cm-‘)
A (nm)
V
800 650 525 440 410 385 330 265 240
12500 15385 19045 22730 24390 25975 30300 37735 41665
v (cm-‘)
_ 15194 19379 22166 24815 26474 30656 37710 41125
1
4. Conclusions
for
Band position observed
4T,,(G)
LETTERS A
[I
] A.J. Ripmeester,
I.C. Ratcliffe. E.J. Dutrtzac and J.L. Jambor. Can. Min. 24 (1986) 435. [2] J.T. Szymanski, Can. Min. 23 (1985) 659. [3] A.Z. Hrynkiewicr. J. Kubisz and D.S. Kulgawczuk. Inorg. Nucl. Chem. 27 ( 1965) 25 13. [4] R.G. Rossman, SE. Grew and W.A. Dollase, Am. Min. 67 (1982) 749. [5 ] Subramanian, Mol. Phys. 54 ( 1985) 415. [ 61 T.R. Hunt and P.R. Ashley, Economic Geology 74 ( 1979) 1613. [ 71 R.G. Rossman, Am. Min. 60 ( 1975) 698. [ R.G. Rossman, Am. Min. 6 I ( 1976) 398. Rosengarten, Mol. Spectrosc. 12( 1964) 3 19.